At the risk of grinching your charitable impulse, don't forget used bookstores and online options.

I am fond of McKays.

http://www.judysbook.com/cities/centreville-va/Books/91403/McKays_Used_Books_Inc_.htm

And for online.

http://www.paperbackswap.com/index.php

It's more "I need this crap out of my house before I move in a few weeks", although paperback swap could work well for the future. Do you store up credits when people want your stuff or do you have redeem within a certain period.